Have your changes had any effect? I have tried changing the strings by putting a new strings file in a Winterboard theme, but for some reason it has no effect. Respringing, wiping cache and changing language to and fro don't help.
I've been googling and asking around a bit, and it seems that other people having this problem did what you suggested, i.e. editing the actual system files as opposed to using Winterboard. So that's what I've done too, now! Still bugs me that I can't figure out how to make Winterboard do it, but ah well!